For vehicles and the like, friction materials such as disc brake pads and brake linings for braking are used.
Such friction materials produce friction with counterpart materials such as a disc rotor and a brake drum to play a role in braking. Therefore, they are required to have not only high friction coefficient and friction coefficient stability but also sufficient strength.
Such friction materials are produced by molding a composition containing a fiber matrix, a binder, a filler, and the like. In particular, the fiber matrix forms the skeleton of friction materials to impart properties for the shear strength of friction materials.
Conventionally, asbestos materials (asbestos) have been used as the fiber matrix for friction materials. Since asbestos was recognized as a material having a harmful effect on human body, alternatives to it have been produced under legal control or the like in each country.
There is proposed a friction material containing different fibers such as inorganic fibers, metal fibers and organic fibers as a fiber matrix alternative to the asbestos material (see for example Patent Document 1). In particular, mineral fibers as an alternative thereto are known to be highly effective in increasing the strength of friction materials.
Mineral fibers are generally produced by melt spinning method. Among mineral fibers produced by melt spinning method, long mineral fibers have poor dispersibility, and the unloosed fibers do not contribute to an improvement in the strength of friction materials. On the other hand, highly dispersible short fibers tend to have a low reinforcing effect. In addition, when the blending amount of mineral fibers is increased, not only the dispersibility may be degraded, but also disadvantages such as an increase in aggressiveness to the opposite material may tend to occur.